Is it 'magical thinking' that the NSA would build a system with the capability to intercept every VPN session traversing the US, and spend ~$500 million to break the DH groups with the mother of all rainbow tables?

So, to GPS. In the original GPS SA it is certainly true that many satellites would have to be using degraded mode to guarantee degradation for a specific point. However, the newer satellites have an additional directional antenna. While primarily for transmitting M-code to a specific region, it could equally be used for transmitting a self-jamming signal.